Truthfully, it is impossible to know for sure the exact method used by John to baptize Jesus. However, given that archaeological evidence indicates that the normal method during the Church’s early years was pouring, I would say that it is likely that the artists’ renderings of the scene depicting John using either a small basin/bowl or a shell are probably how it happened. As you say, we should follow Jesus’ example, and the early Church believed the same way. Since the early Church used pouring as the norm, it is probably because that’s how the Lord was baptized.
Check out 1 Peter 3: 20-22; Exodus 14:15-16, 21-22…Notice that in these prefigurements of Baptism, the ark and crossing the Red Sea, the faithful are not actually plunged into the waters. Check out Joshua 3:14-17, where the faithful are, again, not plunged into the water. This, too, is a prefigurement of the Sacrament of Baptism. In a smaller prefigurement, look to 2 Kings chapter 2, where Elijah parts the water with his mantle so he and Elisha may cross on dry land, and then Elisha parts the waters shortly after. Peter echoes Psalm 69, crying out for God to save him from the waters.
The sacrament is also called “anointing” according to the Catechism 1216, being priestly and royal “as are those who are anointed.” Look to 1 Samuel 10 to see how Samuel anointed Saul. He poured the oil over his head. Look to Leviticus 8:12 to see how Aaron was anointed; oil was poured over his head.
There is more to Baptism than the aspect of death and resurrection, and one must take the whole into account when considering the symbolism, not just the one aspect.
I highly recommend reading the section on Baptism in the Catechism of the Catholic Church. There is deep, rich meaning in the Sacrament, and the CCC will help you begin to explore it.